1. Field of the Invention
The present invention relates to a combined touch sensor and LED (Light-Emitting Diode) driver.
2. Description of the Related Arts
Modern electronic devices often have both a display device to display information and touch sensors to receive input data. There are a variety of types of touch sensor applications, such as touch screens, touch buttons, touch switches, touch scroll bars, and the like. For example, a cellular telephone or personal digital assistant often has a touch screen and a liquid crystal display (LCD) device overlaid with the touch screen.
LCDs typically require a backlight to provide a light source for the LCD display. White LEDs are being used increasingly as the backlight for LCDs. These white LEDs for backlighting LCDs are typically driven by an LED driver that feeds high, constant sink current through the white LEDs to provide constant luminescence, while the anode of the white LED is typically driven by a charge pump circuit.
Touch sensors have a variety of types, such as resistive type, capacitive type, and electro-magnetic type. A capacitive touch screen is coated with a material, typically indium tin oxide, that conducts a continuous electrical current across a sensor. The sensor exhibits a precisely controlled field of stored electrons in both the horizontal and vertical axes of the display to achieve capacitance. The human body is also an electrical device which has stored electrons and therefore also exhibits capacitance. When the sensor's normal capacitance field (its reference state) is altered by another capacitance field, e.g., by the touch with someone's finger, capacitive type touch sensors located at each corner of the touch screen panel measure the resultant distortion in the characteristics of the reference field and send the information about the touch event to the touch screen controller for mathematical processing. There are a variety of types of capacitive touch sensors, including Sigma-Delta modulators (also known as capacitance-to-digital converters (CDCs)), charge transfer type capacitive touch sensors, and relaxation oscillator type capacitive touch sensors.
Because of the small size required in mobile electronic devices such as cellular telephones, LED drivers are sometimes combined with touch sensors on one integrated circuit (IC) chip. In this case, one or more ports of the combined touch sensor and LED driver IC may be used for the touch sensors in one instance and the LED driver in another instance depending upon the settings on the IC. These common, shared ports on the combined touch sensor and LED driver IC are beneficial, because (i) the size of the IC may be reduced and (ii) the same port may be conveniently used with the touch sensor or the LED driver depending upon the user's settings and needs. However, combining the LED driver with touch sensor on one IC with shared ports may present problems due to different operating voltages used in the LED driver and the touch sensor. Touch sensors typically operate on an operating voltage of 1.65-1.95 volt, while LED drivers typically operate on a much higher operating voltage of 3.0-4.3 volt in order to drive the LED. Since the LED driver is fabricated on the same IC as the touch sensor and both the LED driver and touch sensor may be connected to a shared port of the combined touch sensor and LED driver IC, the higher operating voltage of the LED driver may affect the operation of the touch sensor circuit and thereby cause malfunction in the touch sensor circuit or even damage the touch sensor circuit.
Thus, there is a need for a combined touch sensor and LED driver IC without such problems.